The Global Positioning System (GPS) includes 24 satellites that broadcast navigation information in the form of RF signals. GPS receivers process the navigation signals in order to determine precise position, velocity and time and are widely used in both military and civilian applications. Examples of GPS applications include aircraft navigation and landing systems, transportation systems, and weapon systems.
When the GPS malfunctions, errors are introduced into the transmitted navigation signals, thereby resulting in errors in the position, velocity and time calculations performed by receivers. Due to the critical nature of many GPS applications, such errors in the navigation signals can cause significant safety concerns. When the GPS is functioning properly, the navigation signals are characterized as having “integrity” and the GPS position solution meets its advertised accuracy requirements.
Newer GPS receivers are referred to herein as advanced users and include, but are not limited to GPS III receivers, with rigid accuracy and integrity requirements. Specifically, GPS III users require position errors with accuracy better than 1 meter. In order to meet these requirements, it is necessary that users be alerted to conditions that cause a particular accuracy degradation (i.e., a loss of integrity). According to one GPS specification, users must be alerted when the navigation signal's error exceeds the User Range Accuracy (URA) index (i.e., a statistical indicator of the ranging accuracies achievable with the satellite) multiplied out to 4.42 standard deviations. In GPS III, it is required that users be alerted to such a condition within 5.2 seconds of the condition arising (i.e., the required “time to alert” is 5.2 seconds). By contrast, some older GPS receivers, referred to herein as legacy users, have position accuracy requirements on the order of multiple meters and require a time to alert on the order of 6 hours.
Various mechanisms have been developed to monitor the integrity of GPS signals. As one example, a system known as Wide Area Augmentation System (WAAS) provides integrity monitoring by employing several ground stations at known locations over North America. Each ground station receives and processes navigation signals from the GPS satellites in order to calculate its own position. The calculated position is compared to the known position in order to determine the accuracy of the navigation signals. Alternatively or additionally, a satellite monitoring scheme may be used in which each satellite monitors itself or monitors itself and other satellites, as is described in a U.S. Pat. No. 6,603,426 entitled “Satellite Integrity Monitor and Alert” which issued on Aug. 5, 2003.
One technique used to protect GPS users from using hazardously misleading navigation signals is to make the satellite signal unusable to all users. This is achieved by switching the satellite signal to a non-standard code that users do not no how to interpret. However, use of this approach is not optimal since some GPS users, particularly legacy users that have less stringent accuracy requirements, would be denied service from the GPS satellite unnecessarily.
What is needed is a way to alert GPS users to a loss of integrity with sufficient speed to satisfy safety concerns and regulatory standards while keeping the signal usable to users with less stringent requirements. One way to meet this need is to create a new message type for purposes of conveying the integrity alert message. However, with the current GPS messaging scheme, a new message could only be guaranteed to get to users within 30 seconds of the onset of a problem. It is highly advantageous to provide integrity alert data in a manner that does not adversely impact legacy users, either in terms of degradation of service or by requiring an equipment modification or upgrade, since there are an enormous number of legacy receivers in use.